An aircraft registration is a unique alphanumeric string that identifies a civilian aircraft. All ICAO countries require aircraft over a certain weight to be registered with a national aviation authority. Upon registration, the aircraft receives its unique “registration” which must be displayed prominently on the aircraft. Since for airplanes, this is usually the aft fuselage, the registration is often referred to as the tail number.
The aircraft registration is made up of a prefix selected from the countries call sign prefix allocated by the ITU (making the registration a quick way of determining the country of origin) and the registration suffix. Depending on the country of registration, this suffix is a numeric or alphanumeric code and consists of one to five digits or characters respectively.
As aircraft land, it is useful to gather the aircraft's tail number for a variety of purposes, e.g., security, billing for landing, flight status, etc. However, since the tail number is physically written on the aircraft, 100% effective monitoring would require a person to physically read and record each tail number as the aircraft lands. This is virtually impossible to accomplish for a variety of reasons.